1. Field of the Inventions
The inventions generally relate to systems and methods for diagnosing or treating medical conditions.
2. Description of the Related Art
There are many medical treatments which involve instances of cutting, ablating, coagulating, destroying, or otherwise changing the physiological properties of tissue (collectively referred to herein as "tissue modification"). For example, tissue modification can be used to change the electrophysiological properties of tissue. Although treatments that include tissue modification are beneficial, the physiological changes to the tissue are often irreversible and the modification of tissue other than the intended tissue can disable or even kill a patient. Accordingly, physicians must carefully select the tissue that is to be treated in this manner.
One area of medical treatment which involves tissue modification is the ablation of cardiac tissue to cure various cardiac conditions. Normal sinus rhythm of the heart begins with the sinoatrial node (or "SA node") generating a depolarization wave front. The impulse causes adjacent myocardial tissue cells in the atria to depolarize, which in turn causes adjacent myocardial tissue cells to depolarize. The depolarization propagates across the atria, causing the atria to contract and empty blood from the atria into the ventricles. The impulse is next delivered via the atrioventricular node (or "AV node") and the bundle of HIS (or "HIS bundle") to myocardial tissue cells of the ventricles. The depolarization of these cells propagates across the ventricles, causing the ventricles to contract. This conduction system results in the described, organized sequence of myocardial contraction leading to a normal heartbeat.
Sometimes aberrant conductive pathways develop in heart tissue, which disrupt the normal path of depolarization events. For example, anatomical obstacles in the atria or ventricles can disrupt the normal propagation of electrical impulses. These anatomical obstacles (called "conduction blocks") can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called "reentry circuits," disrupt the normal activation of the atria or ventricles. As a further example, localized regions of ischemic myocardial tissue may propagate depolarization events slower than normal myocardial tissue. The ischemic region, also called a "slow conduction zone," creates errant, circular propagation patterns, called "circus motion." The circus motion also disrupts the normal depolarization patterns, thereby disrupting the normal contraction of heart tissue.
The aberrant conductive pathways create abnormal, irregular, and sometimes life-threatening heart rhythms, called arrhythmias. An arrhythmia can take place in the atria, for example, as in atrial tachycardia (AT), atrial fibrillation (AFIB) or atrial flutter (AF). The arrhythmia can also take place in the ventricle, for example, as in ventricular tachycardia (VT).
In treating VT and certain other arrhythmias, it is essential that the location of the sources of the aberrant pathways (called substrates) be located. Once located, the tissue in the substrates can be destroyed, or ablated, by heat, chemicals, or other means of creating a lesion in the tissue. Ablation can remove the aberrant conductive pathway, restoring normal myocardial contraction. The lesions used to treat VT are typically relatively deep and have a large surface area. However, there are some instances where shallower lesions will successfully eliminate VT.
The lesions used to treat AFIB, on the other hand, are typically long and thin and are carefully placed to interrupt the conduction routes of the most common reentry circuits. More specifically, the long thin lesions are used to create a maze pattern which creates a convoluted path for electrical propagation within the left and right atria. The lesions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The lesions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony.
Prior to modifying the electrophysiological properties of cardiac tissue by ablation, or by other means of destroying tissue to create lesions, physicians must carefully determine exactly where the lesions should be placed. Otherwise, tissue will be unnecessarily destroyed. In addition, the heart is in close proximity to nerves and other nervous tissue and the destruction of this tissue will result in severe harm to the patient.
With respect to the treatment of VT, physicians examine the propagation of electrical impulses in heart tissue to locate aberrant conductive pathways. The techniques used to analyze these pathways, commonly called "mapping," identify regions (or substrates) in the heart tissue which can be ablated to treat the arrhythmia. One form of conventional cardiac tissue mapping techniques uses multiple electrodes positioned in contact with epicardial heart tissue to obtain multiple electrograms. The physician stimulates myocardial tissue by introducing pacing signals and visually observes the morphologies of the electrograms recorded during pacing, which this Specification will refer to as "paced electrograms." The physician visually compares the patterns of paced electrograms to those previously recorded during an arrhythmia episode to locate tissue regions appropriate for ablation. These conventional techniques require invasive open heart surgical techniques to position the electrodes on the epicardial surface of the heart.
Conventional epicardial electrogram processing techniques used for detecting local electrical events in heart tissue are often unable to interpret electrograms with multiple morphologies. Such electrograms are encountered, for example, when mapping a heart undergoing ventricular tachycardia (VT). For this and other reasons, consistently high correct identification rates (CIR) cannot be achieved with current multi-electrode mapping technologies. In treating VT using conventional open-heart procedures, the physician may temporarily render a localized region of myocardial tissue electrically unresponsive during an induced or spontaneous VT episode. This technique, called "stunning," is accomplished by cooling the tissue. If stunning the localize region interrupts an ongoing VT, or suppresses a subsequent attempt to induce VT, the physician ablates the localized tissue region. However, in conventional practice, cooling a significant volume of tissue to achieve a consistent stunning effect is clinically difficult to achieve.
Another form of conventional cardiac tissue mapping technique, called pace mapping, uses a roving electrode in a heart chamber for pacing the heart at various endocardial locations. In searching for the VT substrates, the physician must visually compare all paced electrocardiograms (recorded by twelve lead body surface electrocardiograms (ECG's)) to those previously recorded during an induced VT. The physician must constantly relocate the roving electrode to a new location to systematically map the endocardium.
These techniques are complicated and time consuming. They require repeated manipulation and movement of the pacing electrodes. At the same time, they require the physician to visually assimilate and interpret the electrocardiograms. Because the lesions created to treat VT typically have a large volume, the creation of lesions that are improperly located results in a large amount of tissue being destroyed, or otherwise modified, unnecessarily. Additionally, because these techniques do not distinguish between VTs that require a deep lesion, and VTs that can be treated with a more shallow lesion, tissue will be unnecessarily modified when a deep lesion is made to treat VTs that only require a more shallow lesion.
Turning to the treatment of AFIB, anatomical methods are used to locate the areas to be ablated or otherwise modified. In other words, the physician locates key structures such as the mitral valve annulus and the pulmonary veins. Lesions are typically formed that block propagations near these structures. Additional lesions are then formed which connect these lesions and complete the so-called "maze pattern." However, the exact lesion pattern, and number of lesions created, can vary from patient to patient. This can lead to tissue being unnecessarily destroyed in patients who need fewer lesions than the typical maze pattern.
Another issue that often arises in the treatment of AFIB is atrial flutters which remain after the physician finishes the maze procedure. Such flutters are the result of gaps in the lesions that form the maze pattern. The gaps in the lesions must be located so that additional tissue modification procedures may be performed to fill in the gaps. Present method of locating these gaps are, however, difficult and time consuming.
There thus remains a real need for systems and procedures that simplify the process of locating tissue that is intended for cutting, ablating, coagulating, destroying, or otherwise changing its physiological properties.